Structural foam plastics are being developed for appliance, automotive and furniture uses providing lower cost, light-weight products having high utility. Such products are molded from compositions that will foam under molding conditions wherein the gas liberating agent will provide gases in situ, at elevated molding temperatures, that blow the molded composition reducing its density by 2 to 50%.
Such foamable moldable compositions will fill injection molds readily, however, the set-up or cooling time in the mold has been longer than for conventional molding causing molding costs to be higher, offsetting the lower material cost advantages.
It is the objective of the present invention to provide foamable polyblend compositions that can be foamed and molded simultaneously with shorter cycles than conventional foamable polymeric materials.
U.S. Pat. No. 3,268,636 discloses a process for the injection molding of foamed plastic articles, teaching the general concepts of molding foamable plastic using thermoplastics and various blowing agents. U.S. Pat. No. 2,941,964 discloses foamable polystyrene compositions for extrusion using low boiling alipahtic hydrocarbons and chemical compounds as nucleating agents as they release gases. Here, chemical blowing agents alone fail to give low density foamed materials and the aliphatic hydrocarbons are the essential blowing agent to give low density foams if properly nucleated. U.S. Pat. No. 3,231,524 discloses a method for extrusion of low density foamed plastic sheets using volatile aliphatic hydrocarbons injected during extrusion wherein the foam is nucleated with minor amounts (0.3 to 5%) of a carboxylic acid monomer copolymer. The aliphatic hydrocarbon provides an easily extrudable gel that foams on pressure release at the die giving low density foam sheet.
The present invention relates to moldable foamable alloys of relative high density that can be used as structural tough molded articles for furniture, appliances and automotive. Styrenic plastics such as polystyrene, styrene-acrylonitrile copolymers and rubber reinforced styrene polymers are used conventionally for such molded articles having high modulus and toughness along with excellent melt flow properties for molding. Such materials when foamed with aliphatic blowing agents are plasticized with the blowing agents, hence, loose modulus and more importantly have longer set-up times or molding cycles.
It has now been found, unexpectedly, that alloys of high heat distortion interpolymers, in particular alkenyl aromatic-maleic anhydride interpolymers with styrenic polymers, in amounts of 15 to 50% by weight, provide alloys that foam readily with chemical blowing agents to produce fine cell structure yet provide moldable polymeric alloy compositions that have reduced molding cycles and excellent structural engineering properties even though the density has been reduced 2 to 50%.